Incorrectly emulated undocumented ANC instruction #13
Description
As reported at forum6502:
This instruction is called stable, so I'd expect that visual6502 simulates it as described. However, it seems that A doesn't get updated at all (this seems to be incorrect, as ANC should behave almost like AND).
Here's the C code to reproduce the issue (slightly updated version of that posted in the above thread):
#include <stdio.h>
#include "types.h"
#include "perfect6502.h"
#include "netlist_sim.h"
#include <assert.h>
// gcc -Wall -o testANC testANC.c perfect6502.c netlist_sim.c
/*
* $0B ANC #imm
* $2B ANC #imm
*
* ANDs the contents of the A register with an immediate value and then moves bit 7 of A
* into the Carry flag.
*
* See https://csdb.dk/release/?id=198357
*/
int main(void)
{
memory[0] = 0xa9; /* LDA */
memory[1] = 0xff; /* $ff */
memory[2] = 0x38; /* SEC */
memory[3] = 0x0b; /* ANC */ // 0x2b
memory[4] = 0x00; /* $00 */
memory[5] = 0xea; /* NOP */
memory[6] = 0x00; /* BRK */
state_t *state = initAndResetChip();
/* Cycle through the loading of the RESET vector. */
for (int i = 0; i < 16; i++)
step(state);
printf("-- Fetching LDA:\n");
for (int i = 0; i < 2; i++) {
step(state);
chipStatus(state);
}
printf("-- Executing LDA:\n");
for (int i = 0; i < 4; i++) {
step(state);
chipStatus(state);
}
printf("-- Executing SEC:\n");
for (int i = 0; i < 4; i++) {
step(state);
chipStatus(state);
}
printf("-- Executing ANC:\n");
for (int i = 0; i < 4; i++) {
step(state);
chipStatus(state);
}
unsigned char regA = readA(state);
assert(regA == 0x00);
unsigned char flags = readP(state);
assert((flags & 0x01) == 0x00);
printf("-- Executing NOP:\n");
for (int i = 0; i < 4; i++) {
step(state);
chipStatus(state);
}
printf("-- Executing BRK:\n");
for (int i = 0; i < 10; i++) {
step(state);
chipStatus(state);
}
destroyChip(state);
return 0;
}